This invention relates to a low heat-leak system for containing or guiding fluids at cryogenic temperatures.
In the containment or guidance of fluids at cryogenic temperatures, namely, cryogenic fluids, it is important to provide a system that will have a very low rate of heat transmission, that is, heat leak from the surface exposed to ambient atmosphere to the surface exposed to the fluid at cryogenic temperature. Because of the large temperature difference, the thermal driving potential is very high. Heat leak into a fluid at cryogenic temperature is particularly costly and undesirable because of the large amount of work required in achieving the cryogenic temperature, particularly, in liquefying a gas to form a fluid at cryogenic temperature.
The cryogenic temperature range has been identified in publications, and as is used herein extends from 0K. to about 172K. Insulative systems that perform satisfactorily at temperatures above cryogenic temperatures usually do not perform satisfactorily at cryogenic temperatures. At temperatures below the freezing temperature of water, insulative systems have low internal vapor pressure, which creates high potential for atmospheric moisture to enter the system and impair the insulative quality of a system.
In systems for containing or guiding fluids at cryogenic temperatures, heat leak is usually decreased by providing a space of reduced gaseous pressure, that is, a space evacuated of air or gas to some degree to reduce heat transmission by gaseous conduction. The structure necessary varies with the subatmospheric pressure in the space or degree of evacuation. Higher degrees of evacuation require stronger and thicker walls and structures to support the pressure differential between the evacuated space and the ambient atmosphere.
To reduce heat transmission by radiation, the space usually is filled at least in part with radiation shields, a powder or a matrix of solids and voids. A high degree of evacuation is still typically necessary to achieve tolerable rates of heat transmission across the space. The matrix or powder usually contributes somewhat to the heat transmission rate across the space by conduction through the solid portion of the matrix or powder.
What is needed is a system for containing or guiding fluids at cryogenic temperatures wherein low heat leak is attained without a high degree of evacuation and without a high strength structure. This invention satisfies these needs. The invention employs a coherent aerogel to achieve low rates of heat transmission, preferably with gaseous environment pressures higher than used with other materials in the prior art. The coherent aerogel is in a fixed form capable of bearing and transmitting load so that the structure surrounding the aerogel preferably need not support fully the pressure loading imposed by the ambient atmosphere, but can transmit the pressure loading from one external face of the enclosure, through the aerogel, to the other face of the enclosure, thereby balancing the pressure loading of the ambient atmosphere.
Aerogels are water-free gels dried in such a way that the solid matter in the gel remains intact. The resulting solid is an amorphous lattice structure with ultrafine open cells typically consisting of 1 to 5% solid matter. Aerogels have continuous porosity and a microstructure of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 0.01 micrometers. Abundant pores of nanometer size through out the aerogel comprise most of the aerogel's volume.
Inorganic aerogels which have been prepared in coherent form include silica, alumina, zirconia, tungsten, and titanium aerogels, made via the hydrolysis and condensation of the metal alkoxide, for example, tetramethoxy silane, in an alcohol to form an alcogel. The alcogel is dried at supercritical conditions for the alcohol, or at supercritical conditions for a solvent substituted for the alcohol, so as to form a coherent matrix, that is, a coherent aerogel. Alternatively, the alcohol may be replaced with a solvent which is extracted at supercritical conditions for the solvent. Coherent aerogel based on carbon has also been prepared.
Organic aerogels include resorcinol-formaldehyde aerogels formed by the sol-gel polymerization of resorcinol with formaldehyde under alkaline conditions. A typical process is described in U.S. Pat. No. 4,402,927 issued Sep. 6, 1983 to G. von Dardel, hereby incorporated by reference. Another organic aerogel is produced by the sol-gel polymerization of melamine with formaldehyde, introducing a PH change, and following with supercritical extraction, as described in U.S. Pat. No. 5,086,085 issued Feb. 5, 1992 to R. W. Pekala, and hereby incorporated by reference. Representative densities are from about 100 to about 800 kilograms per cubic meter.
All of the aerogels mentioned are capable of being produced in a coherent form, are capable of compressive load bearing, have low densities and display low transmission of heat at atmospheric pressure, and at subatmospheric pressures, notably at low vacuum.